Method of forming a brown coating on steel articles



United States Patent 3,275,536 METHOD OF FORMING A BROWN COATING 0N STEEL ARTICLES John A. McCarthy, Monroeville, Pa., assignor to United States Steel Corporation, a corporation of Delaware No Drawing. Filed Nov. 6, 1962, Ser. No. 235,854 5 Claims. (Cl. 204--56) This invention relates to the coating of metal and, more particularly, to a method for forming a brown coating on steel articles.

The formation of colored coatings (usually oxides) on metal articles has been practiced extensively to improve their appearance and afford added resistance to corrosion. So far as I am aware, however, no quick, simple and safe method is known for producing a brown coating on steel articles. It is accordingly the object of my invention to provide an improved method of forming an attractive brown coating on such articles.

In general terms, my improved method involves electrolytic treatment of the articles to be coated, as anodes, in a hot concentrated caustic electrolyte containing a small addition of one or more of the following: citric, gluconic, glycolic (hydroxyacetic), malic, mandelic, tartaric acids and alkali-metal salts thereof. Such electrolysis, in a few minutes, forms on the articles a hard, highly adherent coating of an oxide having a rich brown color.

More specifically, I first clean and pickle the articles to be coated, using commercially available proprietary alkaline cleaners and a dilute Water solution of sulfuric acid. I then place the articles, connected as anodes, in a Water solution containing from 300 to 900 grams per liter of caustic (sodium or potassium hydroxide) and from 10 to 250 g./l. of any one of the organic hydroxy acids mentioned above or an alkali-metal salt thereof. This electrolyte is preferably heated to a temperature of from 115 to 145 C. and electrolysis is initiated and continued, at a current density of from 0.2 to 7 amperes per square inch, for from 0.5 to minutes. The time required to form the desired coating color is dependent on the concentration and temperature of the reagents and the current density. Cathodes of carbon or stainless steel are used in the electrolyte.

As indicated above, the electrolyte used in the method of the present invention is a caustic solution which, because of the incorporation of a hydroxy acid or an alkalimetal salt thereof, produces uniform, adherent rich brown or bronze oxide coatings on carbon steel and certain alloy steels. The advantages of the process are the attractiveness of the coating, the ease with which a uniform product is reproducibly obtained, the short processing time (1 to 5 minutes), and the improved corrosion resistance as compared to uncoated steel. The process of my invention is an advance over those known heretofore in the attractiveness of the coating and the ease of application and control. The process, compared to those known, is much quicker and, in addition, the composition of the processing solution of this invention is usually simpler (i.e., involves fewer ingredients) or is less concentrated in regard to caustic content (30 to 50% for those solutions versus 80% for certain prior solutions), which means a saving in chemical and heating costs.

Details of my improved process will further appear from the following tabulation of typical examples of electrolyte and plating conditions for carbon steel:

Patented Sept. 27, 1966 Example 1 Sodium hydroxide g./l 500 Hydroxyacetic acid g./l 30 Temperature C Current density a.s.i 3 Time "minutes" 3 Example 11 Sodium hydroxide g./l 500 Sodium tartrate g./L Temperature C 120 Current density a.s.i 1.5 Time minutes 3 Although the most attractive brown coatings are obtained over a wider range of current densities and times with the electrolyte and conditions of Example II, the electrolyte and conditions of Example I are preferred because the b-ath is considerably less susceptible to the production of a gelatinous White precipitate in the bath, probably sodium carbonate.

For most of the combinations of electrolytes and conditions broadly stated above, the processed part will have varying degrees of loose brown oxide on top of the adherent uniform brown oxide coating. For parts with shapes which are easily wiped, this is no great disadvantage, since the underlying coating is sound and of excellent quality. The presence and amount of powder appear to depend upon a complicated relationship between the r-atio of iron sequestrant (hydroxy acid or alkali-metal salt) and caustic content and the conditions of current density and time. With the preferred processing conditions listed in Examples I and II, no surface powder is present on the brown oxide coating.

Desirable brown coatings are formed at treatment intensities of about 0.6 ampere-minutes per square inch or more. (This treatment-intensity figure may be calculated by multiplying current density of the treatment in a.s.i. by the duration of treatment in minutes.) Although brown coatings have been produced at treatment intensities as great as 21 ampere-minutes per square inch, it is desirable for economic reasons to keep this figure within about 10 ampere-minutes per square inch.

The electrolyte exhibits excellent throwing power and uniform color is obtained over irregularly shaped parts with no necessity for auxiliary or specially shaped anodes. The shade of brown produced will naturally differ to some extent depending upon the smoothness of the basic metal, the smoothest parts exhibiting the greatest gloss. All carbon steels and some alloy steels may be coated, but the treatment times and current densities toobtain a given depth of color may be different for two steels widely different in composition. A uniform deep brown may even be obtained on stainless steels of the 400 series (chromium type). Stainless steels of the 300 series (nickel-chrome) will form a brass colored coating on a polished surface and a khaki color on a matte surface, at current densities of about 0.3 ampere per square inch and a treatment time of two minutes.

The corrosion resistance of carbon-steel objects properly coated with brown oxide is equivalent to that of black-oxidized steel articles. Like black oxide coatings, these brown oxide coatings should be oiled after processing to provide added corrosion resistance and gloss. If it is desired to lacquer the coatings, excellent lacquer ad hesion is obtained by treating the parts, after brown oxidizing and water rinsing and prior to l-acquering, in a 3 hot to C.) solution of 5% chromic acid for 15 seconds. This treatment may in any case be desirable, but is not necessary unless the parts are to be lacquered.

Although I have disclosed herein the preferred embodiment of my invention, I intend to cover as Well any change or modification therein which may be made Without departing from the spirit and scope of the invention.

Iclaim:

1. A method of forming an adherent brown coating on steel articles comprising subjecting the articles to electrolysis as anode in a solution consisting essentially of water, from 300 to 900 grams of alkali-metal hydroxide per liter and from 10 to 250 grams per liter of a reagent selected from the group consisting of citric, gluconic, malic, mandelic, glycolic and tartaric acids and alkalimetals salts thereof, while maintaining said solution at a temperature of from to C.

2. A method as defined in claim 1, characterized by effecting said electrolysis at a current density of from 0.2 to 7 amperes per square inch of surface area of said articles.

3. A method as defined in claim 1, characterized by 41 eflecting said electrolysis at an intensity of from 0.6 to 21 ampere-minutes per square inch of the surface area of said articles.

4. A method as defined in claim 1, characterized by said solution containing about 500 grams of hydroxide per liter.

5. A method as defined in claim 1, characterized by said solution containing about grams of said reagent per liter.

References Cited by the Examiner UNITED STATES PATENTS 827,802 8/1906 Hollis 20456 XR 1,386,076 8/1921 Rondelli et a1. 204-56 XR 2,957,812 10/1960 Bongartz et al. 204-56 FOREIGN PATENTS 692,124 6/ 1940 Germany.

JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner. 

1. A METHOD OF FORMING AN ADHERENT BROWN COATING ON STEEL ARTICLES COMPRISING SUBJECTING THE ARTICLES TO ELECTROLYSIS AS ANODE IN A SOLUTION CONSISTING ESSENTIALLY OF WATER, FROM 300 TO 900 GRAMS OF ALKALI-METAL HYDROXIDE PER LITER AND FROM 10 TO 250 GRAMS PER LITER OF A REAGENT SELECTED FROM THE GROUP CONSISTING OF CITRIC, GLUCONIC, MALIC, MANDELIC, GLYCOLIC AND TARTARIC ACIDS AND ALKALIMETALS SALTS THEREOF, WHILE MAINTAINING SAID SOLUTION AT A TEMPERATURE OF FROM 115 TO 145*C. 